Search for a lighter Higgs boson in the Next-to-Minimal Supersymmetric Standard Model

Following the discovery of the Higgs boson with a mass of approximately 125 Ge V at the LHC, many studies have been performed from both the theoretical and experimental viewpoints to search for a new Higgs Boson that is lighter than 125 Ge V. We explore the possibility of constraining a lighter neutral scalar Higgs boson h_1 and a lighter pseudo-scalar Higgs boson a_1 in the Next-to-Minimal Supersymmetric Standard Model by restricting the next-to-lightest scalar Higgs boson h_2 to be the one observed at the LHC after applying the phenomenological constraints and those from experimental measurements. Such lighter particles are not yet completely excluded by the latest results of the search for a lighter Higgs boson in the diphoton decay channel from LHC data. Our results show that some new constraints on the Next-to-Minimal Supersymmetric Standard Model could be obtained for a lighter scalar Higgs boson at the LHC if such a search is performed by experimental collaborations and more data. The potentials of discovery for other interesting decay channels of such a lighter neutral scalar or pseudo-scalar particle are also discussed.     

Implications of the Higgs boson discovery

With the discovery of the Higgs boson by the LHC in 2012, a new era started in which we have direct experimental information on the physics behind the breaking of the electroweak (EW) symmetry. This breaking plays a fundamental role in our understanding of particle physics and sits at the high-energy frontier beyond which we expect new physics that supersedes the Standard Model (SM). In this review we summarize what we have learned so far from LHC data in this respect. In the absence of new particles having been discovered, we discuss how the scrutiny of the properties of the Higgs boson (in search for deviations from SM expectations) is crucial as it can point the way for physics beyond the SM. We also emphasize how the value of the Higgs mass could have far-reaching implications for the stability of the EW vacuum if there is no new physics up to extremely large energies.     

Why Large Hadron Collider?

I discuss LHC physics in the historical perspective of the progress in particle physics. After a recap of the Standard Model (SM) of particle physics, I discuss the high energy colliders leading up to LHC and their role in the discovery of these SM particles. Then I discuss the two main physics issues of LHC, i.e. Higgs mechanism and supersymmetry. I briefly touch upon Higgs and SUSY searches at LHC along with their cosmological implications.     

Discovery of the Higgs boson by the ATLAS and CMS experiments at the LHC

The Standard Model (SM) Higgs boson was predicted by theorists in the 1960s during the development of the electroweak theory. Prior to the startup of the CERN Large Hadron Collider (LHC), experimental searches found no evidence of the Higgs boson. In July 2012, the ATLAS and CMS experiments at the LHC reported the discovery of a new boson in their searches for the SM Higgs boson. Subsequent experimental studies have revealed the spin-0 nature of this new boson and found its couplings to SM particles consistent to those of a Higgs boson. These measurements confirmed the newly discovered boson is indeed a Higgs boson. More measurements will be performed to compare the properties of the Higgs boson with the SM predictions.     

Discovering the Higgs bosons of minimal supersymmetry with muons and a bottom quark

We investigate the prospects for the discovery at the CERN Large Hadron Collider (LHC) of a neutral Higgs boson produced with one bottom quark followed by Higgs decay into a muon pair. We work within the framework of the minimal supersymmetric model. The dominant physics background from the production of b mu(+)mu(-), j mu(+)mu(-), j=g,u,d,s,c, and bbW+W- is calculated with realistic acceptance cuts. Promising results are found for the CP-odd pseudoscalar (A0) and the heavier CP-even scalar (H0) Higgs bosons with masses up to 600 GeV. This discovery channel with one energetic bottom quark greatly improves the discovery potential of the LHC beyond the inclusive channel pp-->phi(0)-->mu(+)mu(-)+X.     

The Higgs sector of supersymmetric theories and the implications for high-energy colliders

One of the main motivations for low-energy supersymmetric theories is their ability to address the hierarchy and naturalness problems in the Higgs sector of the standard model. In these theories, at least two doublets of scalar fields are required to break the electroweak symmetry and to generate the masses of the elementary particles, resulting in a rather rich Higgs spectrum. The search for the Higgs bosons of supersymmetry and the determination of their basic properties is one of the major goals of high-energy colliders and, in particular, the LHC, which will soon start operation. We review the salient features of the Higgs sector of the minimal supersymmetric standard model and of some of its extensions and summarize the prospects for probing them at the LHC and at the future ILC. In memoriam of Julius Wess, 1934–2007.     

Nambu's Nobel Prize,the meson and the mass of visible matter

The electroweak Higgs boson has been discovered in ongoing experiments at the LHC, leading to a mass of this particle of 126 GeV. This Higgs boson mediates the generation of mass for elementary particles, including the mass of elementary (current) quarks. These current‐quark masses leave 98% of the mass of the atom unexplained. This large fraction is mediated by strong interaction, where instead of the Higgs boson the σ meson is the mediating particle. Though already introduced in 1957 by Schwinger, the σ meson has been integrated out in many theories of hadron properties because it had not been observed and was doubted to exist. With the observation of the σ meson in recent experiments on Compton scattering by the nucleon at MAMI (Mainz) it has become timely to review the status of experimental and theoretical researches on this topic.     

The physics of the large hadron collider

The Large Hadron Collider (LHC) at CERN in Geneva, Switzerland, is the most powerful particle accelerator in the world. Its aim is to study the physics of elementary particles at the highest energies accessible to accelerators. It is believed that the Higgs boson (a last particle predicted by the Standard Model that is yet to be found) and the lightest particles of the Minimal Supersymmetric Model should be accessible at the LHC energies. These lectures give a short overview of the physics program and the technological challenges this collider faces.     

Exploring the hyperchargeless Higgs triplet model up to the Planck scale

We examine an extension of the SM Higgs sector by a Higgs triplet taking into consideration the discovery of a Higgs-like particle at the LHC with mass around 125 GeV. We evaluate the bounds on the scalar potential through the unitarity of the scattering matrix. Considering the cases with and without \(\mathbb {Z}_2\)-symmetry of the extra triplet, we derive constraints on the parameter space. We identify the region of the parameter space that corresponds to the stability and metastability of the electroweak vacuum. We also show that at large field values the scalar potential of this model is suitable to explain inflation.     

On mass-shell renormalizability of the massive Yang-Mills theory

Renormalizable theory of electroweak interactions without scalar particles can be constructed by the modifying the Standard Model. One should remove all terms with the scalar field from the Lagrangian in the unitary gauge. The resulting electroweak theory without the Higgs particle is on mass-shell renormalizable and unitary. Thus the experimental non-observation of the Higgs boson will not mean a problem for the concept of renormalizability in quantum field theory but will confirm the scalar free theory.     

Associated production of Higgs boson and t at LHC

One of the future goals of the LHC is to precisely measure the properties of the Higgs boson.The associated production of a Higgs boson and top quark pair is a promising process to investigate the related Yukawa interaction and the properties of the Higgs.Compared with the pure scalar sector in the Standard Model,the Higgs sector contains both scalars and pseudoscalars in many new physics models,which makes the ttH interaction more complex and provides a variety of phenomena.To investigate the ttH interaction and the properties of the Higgs,we study the top quark spin correlation observables at the LHC.     

Finite theories before and after the discovery of a Higgs boson at the LHC

Finite Unified Theories (FUTs) are N = 1 supersymmetric Grand Unified Theories (GUTs) which can be made finite to all‐loop orders, based on the principle of reduction of couplings, and therefore are provided with a large predictive power. Confronting the predictions of SU(5) FUTs with the top and bottom quark masses and other low‐energy experimental constraints a light Higgs‐boson mass in the range M_h ∼ 121–126 GeV was predicted, in striking agreement with the recent discovery of a Higgs‐like state around ∼ 125.5 GeV at ATLAS and CMS. Furthermore the favoured model, a finiteness constrained version of the MSSM, naturally predicts a relatively heavy spectrum with coloured supersymmetric particles above ∼ 1.5 TeV, consistent with the non‐observation of those particles at the LHC. Restricting further the best FUT's parameter space according to the discovery of a Higgs‐like state and B‐physics observables we find predictions for the rest of the Higgs masses and the supersymmetric particle spectrum.     

Associated production of Higgs boson and tt at LHC

One of the future goals of the LHC is to precisely measure the properties of the Higgs boson. The associated production of a Higgs boson and top quark pair is a promising process to investigate the related Yukawa interaction and the properties of the Higgs. Compared with the pure scalar sector in the Standard Model, the Higgs sector contains both scalars and pseudoscalars in many new physics models, which makes the ttH interaction more complex and provides a variety of phenomena. To investigate the ttH interaction and the properties of the Higgs, we study the top quark spin correlation observables at the LHC.     

Physics prospects at the hadron colliders

I start with a brief introduction to the elementary particles and their interactions, Higgs mechanism and supersymmetry. The major physics objectives of the Tevatron and LHC colliders are identified. The status and prospects of the top quark, charged Higgs boson and superparticle searches are discussed in detail, while those of the neutral Higgs boson(s) are covered in a parallel talk by R.J.N. Phillips at this workshop.     

What can we learn from the total width of the Higgs boson?

As one of the key properties of the Higgs boson, the Higgs total width is sensitive to the global profile of the Higgs boson couplings, and thus new physics would modify the Higgs width. We investigate the total width in various new physics models, including various scalar extensions, composite Higgs models, and the fraternal twin Higgs model. Typically, the Higgs width is smaller than the standard model value due to mixture with other scalars if the Higgs is elementary, or curved Higgs field space for the composite Higgs. On the other hand, except for the possible invisible decay mode, the enhanced Yukawa coupling in the two Higgs doublet model or the exotic fermion embeddings in the composite Higgs could enhance the Higgs width greatly. The precision measurement of the Higgs total width at the high-luminosity LHC can be used to discriminate certain new physics models.     

Signatures of extra dimensions from upsilon decays with a light gaugephobic Higgs boson

We explore non-standard Higgs phenomenology in the gaugephobic Higgs model in which the Higgs can be lighter than the usually quoted current experimental bound. The Higgs propagates in the bulk of a 5D space–time and Electroweak Symmetry Breaking occurs by a combination of boundary conditions in the extra dimension and an elementary Higgs. The Higgs can thus have a significantly suppressed coupling to the other Standard Model fields. A large enough suppression can be found to escape all limits and allow for a Higgs of any mass, which would be associated with the discovery of W^′ and Z^′ Kaluza–Klein resonances at the LHC. The Higgs can be precisely discovered at B-factories while the LHC would be insensitive to it due to high backgrounds. In this Letter we study the Higgs discovery mode in (3S), (2S), and (1S) decays, and the model parameter space that will be probed by BaBar, Belle, and CLEO data. In the absence of an early discovery of a heavy Higgs at the LHC, A Super-B factory would be an excellent option to further probe this region.     

Probing the Higgs field using massive particles as sources and detectors

In the standard model, all massive elementary particles acquire their masses by coupling to a background Higgs field with a non-zero vacuum expectation value. What is often overlooked is that each massive particle is also a source of the Higgs field. A given particle can in principle shift the mass of a neighboring particle. The mass shift effect goes beyond the usual perturbative Feynman diagram calculations which implicitly assume that the mass of each particle is rigidly fixed. Local mass shifts offer a unique handle on Higgs physics since they do not require the production of on-shell Higgs bosons. We provide theoretical estimates showing that the mass shift effect can be large and measurable, especially near pair threshold, at both the Tevatron and the LHC. PACS 14.80.Bn; 13.40.Dk     

Partially (in)visible Higgs decays at the LHC

Both ATLAS and CMS have reported a discovery of a Standard Model-like Higgs boson H   of mass around 125 GeV. Consistency with the Standard Model implies the non-observation of non-SM-like decay modes of the newly discovered particle. Sensitivity to such decay modes, especially when they involve partially invisible final states is currently beyond scrutiny of the LHC. We systematically study such decay channels in the form of H→AA→jets+missing energy$H\to AA\to \text{jets}+\text{missing energy}$, with A   a light scalar or pseudo-scalar, and analyze to what extent these exotic branching fractions can be constrained by direct measurements at the LHC. While the analysis is challenging, constraints as good as BR?10%$\mathrm{BR}?10\text{\%}$ can be obtained.     

Physics prospects at the hadron colliders

I start with a brief introduction to the elementary particles and their interactions, Higgs mechanism and supersymmetry. The major physics objectives of the Tevatron and LHC colliders are identified. The status and prospects of the top quark, charged Higgs boson and superparticle searches are discussed in detail, while those of the neutral Higgs boson(s) are covered in a parallel talk by R.J.N. Phillips at this workshop.     

Spin correlations in the Drell–Yan process,parton entanglement,and other unconventional QCD effects

We review ideas on the structure of the QCD vacuum which had served as motivation for the discussion of various non-standard QCD effects in high-energy reactions in articles from 1984 to 1995. These effects include, in particular, transverse-momentum and spin correlations in the Drell–Yan process and soft photon production in hadron–hadron collisions. We discuss the relation of the approach introduced in the above-mentioned articles to the approach, developed later, using transverse-momentum-dependent parton distributions (TDMs). The latter approach is a special case of our more general one which allows for parton entanglement in high-energy reactions. We discuss signatures of parton entanglement in the Drell–Yan reaction. Also for Higgs-boson production in pp$pp$ collisions via gluon–gluon annihilation effects of entanglement of the two gluons are discussed and are found to be potentially important. These effects can be looked for in the current LHC experiments. In our opinion studying parton-entanglement effects in high-energy reactions is, on the one hand, very worthwhile by itself and, on the other hand, it allows to perform quantitative tests of standard factorisation assumptions. Clearly, the experimental observation of parton-entanglement effects in the Drell–Yan reaction and/or in Higgs-boson production would have a great impact on our understanding how QCD works in high-energy collisions.